Many optical, electronic, and other technological fields require the controllable deposition of thin films upon a substrate. A number of processes are known for depositing such thin films. In evaporation processes, for example, a mass of material is heated so that the vapor pressure of the heated material causes it to evaporate from the surface of the material and deposit on a cooler adjacent substrate, forming the thin film.
Sputtering is another technique for depositing a layer of a deposited material onto a surface of a substrate. In sputtering, energetic ions are directed against the surface of a water-cooled plate target formed of the material to be deposited. The energetic ions dislodge and eject atoms of the target material from the surface of the plate target by a mechanical interaction. Some of the ejected atoms deposit onto the surface of the substrate, producing the deposited layer. While the sputtering plate target may be heated to a degree, the sputtering process is distinct from thermally based evaporation processes wherein the atoms are expelled from the surface of material source by thermal energy. The sputtering plate target is produced as a solid plate or piece by one of several techniques.
The target initially has a planar surface. As sputtering proceeds, experience has shown that the removal from the plate target surface of the material to be deposited is not uniform. Instead, the initially planar surface of the plate target tends to become dished in profile, corresponding somewhat to the energy and mass distribution of the sputtering beam. As the surface of the plate target becomes ever more dished, the spatial distribution of the ejected atoms changes. The result is that the deposition on the surface of the substrate varies over time. This variation is undesirable, because in many cases the deposition must be highly precise in respect to the composition and thickness of the deposited layers.
The dishing of the plate target surface is also undesirable economically, because eventually the dished target surface punches through the bottom of the sputtering plate target, when only a fraction of the material of the sputtering plate target has been sputtered. The remainder of the sputtering plate target is discarded or recycled at a substantial reduction in value. Because many types of specialty sputtering plate targets are expensive, this material wastage represents a significant addition to the cost of the final-product.
There is a need for an improved approach to sputtering that reduces or, ideally, eliminates the technical and economic problems associated with the present sputtering approach. The present invention fulfills this need, and further provides related advantages.